The hematopoietic system is characterized by a phenotypic and functional diversity of multiple lineages derived from a common stem cell. Different classes of genes that are critical for the development, proliferation, and function of stem cells, progenitor cells and terminally differentiated lineages have thus far been recognized. These include transcriptional factors (Ness, S. A. and Engel, J. D., Curr. Opin. Genet. Dev. 4:718-724 (1994); Dorshkind, K., Cell 79:751-753 (1994); Scott, E. W. et al., Science 265:1573-1577 (1994)), an array of signaling molecules such as kinases and phosphatases (Hunter, T., Cell 80:225-236 (1995); Taniguchi, T., Science 288:251-255 (1995); Schulz, L. D. et al., Cell 73:1445-1454 (1993)), adhesion molecules (Clark, E. A. and Brugge, J. S., Science 268:233-239 (1995)), and an increasingly complex network of cytokines and cytokine receptors (Kishimoto, T. et al., Cell 76:253-262 (1994); Ihle, J. N. et al., Annul Rev. Immunol. 13:369-398 (1995)). A need exists to further identify hematopoietic-specific proteins and their functions.